https://help.ubuntu.com/lts/serverguide/backup-shellscripts.html
https://alvinalexander.com/linux/linux-crontab-file-format-example
https://crontab.guru/every-3-minutes

To watch the contents of a directory change, you could use

watch -d ls -l
for more: http://beerpla.net/2007/08/04/watch-a-useful-linux-command-you-may-have-never-heard-of/

Select:
https://crontab-generator.org/

 

 

https://serverfault.com/questions/258489/how-to-run-cron-job-on-a-specific-hour-every-day

# +--------- Minute (0-59)                    | Output Dumper: >/dev/null 2>&1
# | +------- Hour (0-23)                      | Multiple Values Use Commas: 3,12,47
# | | +----- Day Of Month (1-31)              | Do every X intervals: */X  -> Example: */15 * * * *  Is every 15 minutes
# | | | +--- Month (1 -12)                    | Aliases: @reboot -> Run once at startup; @hourly -> 0 * * * *;
# | | | | +- Day Of Week (0-6) (Sunday = 0)   | @daily -> 0 0 * * *; @weekly -> 0 0 * * 0; @monthly ->0 0 1 * *;
# | | | | |                                   | @yearly -> 0 0 1 1 *;